Anti-bounce device for a mass striking a target element

ABSTRACT

An anti-bounce device for a mass striking a target element, with the device including a counter mass and a linking arrangement for holding the counter mass spaced from the mass and a braking arrangement for delaying the counter mass so as to put the counter mass in phase opposition with the mass as the mass bounces after impact thereby dissipating any bouncing force. The linking arrangement may take the form of a volume of fluid in a chamber provided between the mass and counter mass, and the linking arrangement and braking arrangement may also include a layer of a damping material or deformable or flexible elements acting by friction.

BACKGROUND OF THE INVENTION

The object of the present invention is an anti-bounce device for a massstriking a target element and generating seismic impulses.

The device according to the invention is particularly useful forachieving impact seismic sources which generate in the ground seismicwaves through the percussion of a moving mass against a target elementwhich it is hurled at by the effect of gravity and/or motive means. Apercussion source for seismic prospecting on land comprising a massfalling along a guiding element towards a target element anchoredagainst the ground surface is described in French Patent No. 2,398,316.Another percussion source, more particularly adapted for being used in awell bore, is described in French Patent No. 2,552,553. The use of motor means for throwing a mass against a target element anchored in a wellis described in French Patent No. 2,558,601.

When the mass drops freely, it generally bounces after its impactagainst the associated target element and it then comes back and strikesit once or several times with decreasing strengths. The "signature" ofthe seismic source, i.e the strength of the emitted impulses , thuscomprises in this case several secondary impulses of decreasingamplitudes following the main impulse and this has the effect ofdisturbing the seismic recordings corresponding to the seismicreflections of the emitted impact pulse on the subsoil discontinuities.

Through French Patent No. 2,509,052, a device is known which avoids themultiple shocks on a target element, connected with the ground surface,of a mass guided in its fall by guiding means, these multiple shocksbeing due to the bounces of the mass after its first impact. This deviceessentially comprises a deformable element fastened to the guiding meansand control means adapted for applying the deformable element againstthe side wall of the mass after its first bounce thereby immobilizing itbefore it falls down again. The control means comprise for example amobile element which is radially moved by the action of a hydraulic jackand an impact detector delivering a control signal resulting in thetripping of the jack.

This device very efficiently blocks any further fall of the mass but,due to its transverse position perpendicular to the longitudinal axis ofthe guiding means, its space requirement would not be compatible withthe generally limited dimensions of the drill holes where the wellseismic sources are taken down.

Through published French Patent Application 2,631,458 an anti-bouncedevice is known which is well-adapted to the cases where the lateralspace around the direction of fall is limited, and which ischaracterized by the use of electromagnetic means in order to exert anintermittent attractive face on the mass. As a result the mass sticks tothe target element that it has just struck at the end of the fall. Sucha device is suitable to any application where it is possible to useelectromagnetic means and to connect them with feeding cables to aelectric source.

SUMMARY OF THE INVENTION

The device according to the invention avoid the multiple shocks of amass striking a target element without having to use magnetic orelectromagnetic forces to stop the mass after its impact. By the use ofthe device, it is possible to obtain very short seismic pulses which arefavorable in particular to seismic prospecting.

The present invention is characterized by a counter-mass which is placedbehind the mass in relation to the direction of displacement towards thetarget element, the counter-mass being movable in relation to the mass,linking means to hold the mass and the counter-mass spaced apart duringtheir moving in the same direction towards the target element, as wellas braking means selected according to the respective features of themass and of the counter-mass, in order to delay the latter in relationto the mass whereby their motions are substantially be in opposition ofphase and act against each other.

The linking means and the braking means comprise, for example, a layerof a damping material the stiffness of which is selected in order tosubstantially cancel the back motion of the mass after its impactagainst the target element.

The linking means may comprise a volume of fluid contained in a chamberprovided between the mass and the counter-mass, and the braking means,at least one passageway with a calibrated diameter allowing an outflowof the fluid out of the chamber under the action of the inertia force ofsaid counter-mass.

The linking means and the braking means may also comprise at least oneflexible part fastened to the mass or the counter-mass and holding theother one by friction. This flexible part is for example a deformablesleeve or a set of spring leaves leaning against the side wall of themass.

It has been possible to verify from experience that, through anappropriate selection of the damping layer between mass andcounter-mass, of the deformable sleeve or of the section of the outletports of the braking fluid, it was practically possible to suppress anybounce of the mass and this in a definitely repetitive way. Theachievement and the implementation of an impact well seismic source iseasier with such a constituted mass.

According to a possible application procedure, the target element isfirmly attached to an extended body associated with retractableanchoring means in order to connect the extended body with the walls ofa well or a drill hole, the mass is equipped with a hold for retractableholding means and it can be moved within the body between a lowerposition corresponding to the impact against the target element and ahigher position where it is distant from the latter.

It is possible to use retractable holding means to displace the mass inrelation to the body, the displacement stroke thereof being sufficientto convey the mass between its lower position and its higher positionand thus to connect the holding means to a lifting device.

The initial spacing between the mass and the counter-mass is simplyrestored, whether the damping material recovers its normal volume aftereach shock, or by traction on the counter-mass by the retractableholding means. The traction forces are sufficient to overcome thefriction forces of the flexible parts or else to restore the initialvolume of fluid in the chamber between the mass and the counter-mass.

BRIEF DESCRIPTION OF THE DRAWINGS

Other features and advantages of the method and of the device accordingto the invention will be clear from reading the description ofembodiments given by way of non limitative examples with reference tothe appended drawings in which :

FIG. 1 shows an embodiment of the invention where the linking means andthe braking means comprise a layer of a damping material;

FIG. 2 shows a mass equipped with linking means and braking meanscomprising lateral springs rubbing against a part of the mass;

FIG. 3 shows a mass equipped with hydraulic linking and braking means;and

FIG. 4 shows a first example of an association of the mass and of theanti-bounce device with a unit allowing to apply the impulse stresses tothe walls of a well.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The anti-bounce device according to the invention is associated with amass 1 adapted for being thrown against an impact wall 2. It can, forexample, be a mass accelerated by the gravitational force and/or anymotor means. It comprises a counter-mass 3, means for linking the massand the counter-mass and means for braking the latter. According to theembodiment of FIG. 1, the linking means and the braking means consist ofa layer 4 made of a damping material which is inserted between the mass1 and the counter-mass 3 and sticks to them. The layer 4 holds the massand the counter-mass at a given distance from one another. When the massis stopped in its motion by the impact wall 2 and bounces back, thecounter-mass 3 continues its movement forward while it is slowed down bythe damping layer 4. The mass and the counter-mass are then out of phaseand act against each other. The ratio of both masses, the thickness ofthe damping layer and the quality of the damping material are selectedso that the kinetic energy of the backward motion of one besubstantially equal to the kinetic energy which is kept by the otherone. In actual practice, it is possible to totally suppress the bouncesof the mass 2.

According to the embodiment of FIG. 2, the linking means and the brakingmeans consist of a deformable sleeves that is, for example, fastened tothe counter-mass 3 and fitted in the end of the mass. The frictionforces which the sleeve 5 exerts on the side wall of mass 1 aresufficient to keep it apart from the counter-mass 3. The inertia forceof counter-mass 3 is still sufficient to overcome the friction forcesand, as previously, the counter-mass 3 continues its motion forwardswhile the mass 1 goes backwards after the shock against the impact wall2. Here again, the bouncing of the mass 1 is practically annihilated.The deformable sleeve 5 may also be replaced by one or several flexibleleaves that are, for example, fixed to counter-mass 3 and laterallygripping mass 2.

According to the embodiment of FIG. 3, the linking means consist of avolume of fluid, for example, oil, contained in a chamber providedbetween the mass 1 and the counter-mass 3. To that effect, a cylindricalcavity 6 closed by a cover 7 is provided in mass 1, on the side oppositeto the impact wall 2. The counter-mass 3 is equipped with a rod 8entering the cavity by way of an opening through cover 7, and extendedby a piston 9 the section of which is adapted for the section of cavity6 that is equipped with filler joints 10 on its circumference. One orseveral parts or channels 11 with a calibrated section pass through thepiston 9 and connect the parts of the chamber on either side of piston 9together. The braking force is exerted on counter-mass 3 by thelamination of the liquid through the ports or channels 11. When mass 1strikes wall 2, its bouncing force is again annihilated by the remaininginertia force of counter-mass 3.

The restoring of the initial spacing between the mass and thecounter-mass is obtained with the embodiments of FIGS. 2 and 3, byexerting a traction on counter-mass 3. To this effect, the counter-masscan be equipped with a hooking head T for a pulling device like the oneshown in FIG. 4.

According to the implementation of FIG. 4, mass 1, which is fitted withthe anti-bounce device according to one of the previously describedembodiments, is adapted for striking a target element 11 linked with thewall of a well or a drill hole 12.

Mass 1 moves following a straight line within an extended body 13 thatis taken down in the well 12 at the end of an electric carrying cable 14comprising supply lines. Towards its end opposite cable 14, the body 13is associated with anchoring skids 15. These skids are fixed at the endof the rods 16 of jacks (not shown) which are radially arranged in acompartment 17 containing a hydraulic operating system. Such a hydraulicsystem is described in the previously cited French Patent No. 2,558,601.

Mass 1, equipped with its anti-bounce device, is displaced from animpact position in contact with target element 11 into a loadingposition by lifting means. These means comprise a rigid rod 18 linked tocable 14 by a mechanical and electrical connector 19. The rod 18 crossesthe upper terminal wall 20 of the body through an opening and inside, inthe cavity 21 where the mass moves, it supports retractable fasteningmeans. These means comprise a rigid support 22. Two hooks 23 can swivelon pins 24 that are fastened to support 22 between a proximity positionand a spacing position (represented with dashed lines on FIG. 4). On itsupper side, counter-mass 3 comprises a head 25 equipped with a circulargroove 26 into which the tips of hooks 23 can fit in the proximityposition. Thus, they make

mass 3 interdependent from fastening support 22. The swiveling of hooks23 towards their spacing position is obtained, for example, through theexcitation of electromagnets 27 with mobile cores 28. These cores areradially arranged in the swiveling plane of hooks 23 and linked to theseones. A return spring 29 tends to hold hooks 23 in their proximityposition.

The excitation of the electromagnets 27 is provided by conductors 30going through the axis of the rigid rod 18 and linked within connector19 to conducting lines of the electric carrying cable 14.

Other electric conductors (not shown) allow the transmission of electriccurrents and of control signals to the hydraulic system (not shown)operating jacks 15.

At the opposite end of electric carrying cable 14, the conductors 30 areconnected to the electric power source 31 by a switch 32. Theirconnection is such that, in a first position of switch 32, theelectromagnets 27 move the hooks 23 away from one another. In the secondposition of switch 32, the same electromagnets 27 drive the hooks 23back towards their proximity position.

Since the seismic source is anchored in the well through the spacing ofthe anchoring skids 15,.switch 32 is placed in its second position andcable 14 is loosened so that the rigid support 22 goes down along cavity21 towards counter-mass 3 which is in its lower position against targetelement 11. While contacting the head 25, the hooks 23 move away fromone another and fit into groove 26 of the latter.

Through a traction on electric carrying cable 14, the support 22 and themass-counter-mass unit which is coupled to it are taken up to a highposition. At the time selected for the release, switch 32 is placed inits first position. Electromagnets 27 separate hooks 23 which keep themass. 10 Once released, the mass falls along cavity 18 and stronglystrikes the target element.

The anti-bounce device according to any one of the embodiments shown inFIGS. 1-3 suppresses all the secondary impacts. The obtained sharperimpact force is transmitted to the formations around the well by theanchoring skids 15.

It is also possible to use the lifting means described in French PatentNo. 2,590,994, where the uncoupling of the mass-counter-mass unit isprovided at the end of the return stroke by contacting hooks such ashooks 23 against supporting walls interdependent from the body of theseismic source, the lifting force exerted from the surface on the cablesupporting the source being sufficient for the hooks to separate fromone another and releases mass 1.

I claim:
 1. An anti-bounce device for a seismic source for generationseismic pulses in a bore hole by impacting a target element in contactwith the earth by a mass dropping from a first position, the anti-bouncedevice comprising a counter-mass for preventing a bouncing of the massmeans for loosely holding and integrally connecting the counter-mass ata predetermined distance from said mass and behind the same as viewed ina dropping direction, and braking means for delaying the movement of thecounter mass with respect to the mass after impact thereby putting themass and counter mass in phase opposition, whereby secondary impacts ofthe mass on said target element are suppressed.
 2. A seismic source forgenerating seismic pulses in a bore hole by impacting a target elementin contact with the earth with a mass dropping from a first position,said seismic source being provided with an anti-bounce device forpreventing any bouncing of said mass and comprising a counter-mass, andloose holding and integrally connecting means for holding by frictionthe counter mass at a limited distance spaced from said mass and behindthe same with respect to a dropping direction, a friction force isselected for delaying the counter-mass movement with respect to the massafter impact thereof thereby putting the mass and the counter-mass inphase opposition and bringing the same closer to each other, wherebysecondary impacts of the mass on the target element are suppressed.
 3. Adevice as claimed in claim 2, wherein said loose holding means includesspring leads leaning against a side wall of the mass.
 4. A device asclaimed in claim 2, wherein said loose holding means includes adeformable sleeve laterally rubbing against the mass.
 5. A seismicsource for generating seismic pulses in a bore hole by impacting atarget element in contact with the earth with a mass dropping from afirst position, said target element being interdependent from anelongated body associated with retractable anchoring means of theseismic source for coupling the elongated body with walls of the borehole, said mass being movable within the elongated body between thefirst position and a lower impact position spaced from said firstposition, and an anti-bounce device comprising a counter mass, holdingmeans for loosely holding and integrally connecting the counter mass ata distance spaced from the mass and behind the same with respect to adropping direction, and braking means for delaying the counter-massmovement with respect to the mass after impact thereof thereby puttingthe mass and the counter-mass in phase opposition and bringing the samecloser to each other, whereby secondary impacts of the mass on thetarget element are suppressed.
 6. A seismic source as claimed in claim5, wherein the holding means are movable in relation to the elongatedbody and have a displacement stroke sufficient to displace the massbetween the lower impact position and the first position.
 7. A device asclaimed in claim 6, wherein the holding means are connected to a liftingdevice.